.\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_prep_openat2 3 "March 13, 2022" "liburing-2.2" "liburing Manual"
.SH NAME
io_uring_prep_openat2 \- prepare an openat2 request
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/stat.h>
.B #include <fcntl.h>
.B #include <linux/openat2.h>
.B #include <liburing.h>
.PP
.BI "void io_uring_prep_openat2(struct io_uring_sqe *" sqe ","
.BI "                           int " dfd ","
.BI "                           const char *" path ","
.BI "                           int " flags ","
.BI "                           struct open_how *" how ");"
.PP
.BI "void io_uring_prep_openat2_direct(struct io_uring_sqe *" sqe ","
.BI "                                  int " dfd ","
.BI "                                  const char *" path ","
.BI "                                  int " flags ","
.BI "                                  struct open_how *" how ","
.BI "                                  unsigned " file_index ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_prep_openat2 (3)
function prepares an openat2 request. The submission queue entry
.I sqe
is setup to use the directory file descriptor
.I dfd
to start opening a file described by
.I path
and using the open flags in
.I flags
and using the instructions on how to open the file given in
.IR how .

For a direct descriptor open request, the offset is specified by the
.I file_index
argument. Direct descriptors are io_uring private file descriptors. They
avoid some of the overhead associated with thread shared file tables, and
can be used in any io_uring request that takes a file descriptor. To do so,
.B IOSQE_FIXED_FILE
must be set in the SQE
.I flags
member, and the SQE
.I fd
field should use the direct descriptor value rather than the regular file
descriptor. Direct descriptors are managed like registered files.

If the direct variant is used, the application must first have registered
a file table using
.BR io_uring_register_files (3)
of the appropriate size. Once registered, a direct accept request may use any
entry in that table, as long as it is within the size of the registered table.
If a specified entry already contains a file, the file will first be removed
from the table and closed. It's consistent with the behavior of updating an
existing file with
.BR io_uring_register_files_update (3).
Note that old kernels don't check the SQE
.I file_index
field, which is not a problem for liburing helpers, but users of the raw
io_uring interface need to zero SQEs to avoid unexpected behavior.
If
.B IORING_FILE_INDEX_ALLOC
is used as the
.I file_index
for a direct open, then io_uring will allocate a free direct descriptor in
the existing table. The allocated descriptor is returned in the CQE
.I res
field just like it would be for a non-direct open request. If no more entries
are available in the direct descriptor table,
.B -ENFILE
is returned instead.

These functions prepare an async
.BR openat2 (2)
request. See that man page for details.

.SH RETURN VALUE
None
.SH ERRORS
The CQE
.I res
field will contain the result of the operation. See the related man page for
details on possible values. Note that where synchronous system calls will return
.B -1
on failure and set
.I errno
to the actual error value, io_uring never uses
.IR errno .
Instead it returns the negated
.I errno
directly in the CQE
.I res
field.
.SH NOTES
As with any request that passes in data in a struct, that data must remain
valid until the request has been successfully submitted. It need not remain
valid until completion. Once a request has been submitted, the in-kernel
state is stable. Very early kernels (5.4 and earlier) required state to be
stable until the completion occurred. Applications can test for this
behavior by inspecting the
.B IORING_FEAT_SUBMIT_STABLE
flag passed back from
.BR io_uring_queue_init_params (3).
.SH SEE ALSO
.BR io_uring_get_sqe (3),
.BR io_uring_submit (3),
.BR io_uring_register (2),
.BR openat2 (2)
